(a) Field of the Invention
This invention relates to an optical surface profile measuring device for contactlessly measuring such minute variations as the surface roughness or step height of objects.
(b) Description of the Prior Art
With the recent remarkable development of the precise working technique, products and parts having minute profiles worked on the surfaces have been increasing. Such surfaces as LSI patterns, diffraction gratings, optical discs and roughness standard pieces can be said to be typical examples having regular minute profiles. Also, step heights made by etching on silicon wafers, cross-sections of groove-shaped scars on aluminum surfaces and mirror surfaces made by superprecisely working steel surfaces can be said to be minute profiles. For example, the surface roughness of a surface super-precisely worked with a diamond bit has attained submicrons. Such surface roughness as on a laser disc, magnetic tape or film is also of the same dimensional order. Such surface roughness and surface step height variations are measured today substantially with a contact needle type measuring device.
However, most of these products and parts are finished goods and are desired to be able to have minute profiles of the surfaces measured without being scarred. Further, the measured results are often utilized as reference data in the case of the work and measurement in the next stage or as data necessary for the transaction of the products. Therefore, in order to meet such needs, contactless type measuring devices are being developed. Considered to be most practical today among them is an optical type. Various types of optical measuring devices are considered. Measuring devices applying a focus detecting type are noted as having the possibility of improving the measurement precision and making the device small. Devices applying a critical angle method and an astigmatism method are among them.
First of all, a device using the critical angle method is shown, for example, in FIG. 1. Here, an infrared laser beam from a laser diode 1 is projected onto a sample 6 through a collimator lens 2, polarization beam splitter 3, 1/4-wave plate 4 and an objective lens 5. The reflected beam enters a critical angle prism 8 or 9 through the objective lens 5, 1/4-wave plate 4, polarization beam splitter 3 and beam splitter 7. The beam reflected by the critical angle prism 8 or 9 enters respectively two photodiodes 10 and 11 or 12 and 13 (See FIG. 2) in the formation. In case the measured surface of the sample 6 is a focus position of the objective lens 5, the beam reflected by the measured surface will be made a parallel beam by the objective lens 5 and enter the critical angle prism 8 or 9. At this time, if the critical angle prism 8 or 9 is so set that the incident beam may be incident just near the critical angle, a beam of the same light amount will reach the respective two photodiodes 10 and 11 or 12 and 13 as shown in FIG. 2B. In case the measured surface is in a position nearer to the objective lens 5 than the focus position, the reflected beam will pass through the objective lens 5, will then become a dispersed beam and will be incident upon the critical angle prism 8 or 9. At this time, as the angle of incidence is different on both sides of the optical axis, the part of the beam on the side not meeting the conditions of total reflection will go out of the prism 8 or 9, but the part of beam on the side meeting the conditions of total reflection will be totally reflected and therefore, as shown in FIG. 2A, only a small amount of radiation will reach the photodiode 10 or 12 but a sufficient amount of radiation will reach the photodiode 11 or 13. In case the measured surface is in a position farther from the objective lens 5 than the focus position, contrary to the above, as shown in FIG. 2C, a sufficient amount of radiation will reach the photodiode 10 or 12 but only a small amount of radiation will reach the photodiode 11 or 13. Therefore, when the sample 6 is moved and scanned in the directions indicated by the arrows in FIG. 1 while reading out the output differences between the respective two diodes 10 and 11 or 12 and 13, the surface roughness and fine step height of the measured surface will be able to be measured.
As a surface profile measuring device using the astigmatism method, there is a device shown, for example, in FIG. 3. Here, a laser beam from a laser light source 14 enters a polarization beam splitter 16 through a spatial filter 15 and is then projected onto a sample 19 through a 1/4-wave plate 17 and an objective lens 18. The reflected beam enters a cylindrical lens 21 or 22 through the objective lens 18, 1/4-wave plate 17, polarization beam splitter 16 and beam splitter 20. The beam is collected so as to produce an astigmatism by the cylindrical lens 21 or 22 an enters a detector 23 or 24 (See FIG. 4) consisting respectively of four photodiodes 25, 26, 27 and 28 in the formation. In this optical system, as there is an astigmatism, in case a spot image from the laser light source is incident upon the measured surface of the sample 19, the shape of the spot image formed on the detector 23 or 24 after it is reflected on the measured surface will be deformed as shown in FIGS. 4A, 4B and 4C in front and rear of the focus. When the image is detected by the photodiodes 25, 26, 27 and 28 and is calculated as (V.sub.25 +V.sub.27)-(V.sub.26 +V.sub.28) and, while reading the values, the sample 19 is moved in the directions indicated by the arrows to be scanned, the surface roughness and fine step height of the measured surface will be able to be measured, where V.sub.25, V.sub.26, V.sub.27 and V.sub.28 represent respectively the output of the detectors 25, 26, 27 and 28.
Now, in such optical surface profile measuring device using the critical angle method as is mentioned above, a high resolving power of a maximum of about 1 .mu.m. can be attained but it is only in a range of about .+-.1 .mu.m. at most that there is a linearity in the relation between the step height change and the output. Therefore, there have been problems that, in the case of the measurement, first of all, the detecting head must be set at a range of only 2 [2m., that, if there are irregularities of more than .+-.1 .mu.m. on the surface of the object to be measured, the measured surface will be out of the measuring range and will not be able to be automatically measured and that such an optical surface profile measuring device using the astigmatism method as is mentioned above has a wide measuring range (more than several tens of .mu.m.) but is low in resolving power.
On the other hand, in such optical surface profile measuring devices using the critical angle method as is mentioned above, as a method of expanding the measuring range, there is a method wherein the objective lens is replaced with a lens of a small NA, deep focal depth and low magnification. However, in such conventional objective lens replacing system as, for example, a turret system, the objective lens part becomes considerably large, the center is likely to be displaced and it is difficult to keep the parfocal at a required precision. Even in a system of replacing a single objective lens, it is difficult to maintain the center at a high precision and to keep the parfocal at a required precision. The work of replacing the objective lens is complicated. Particularly it is very difficult to make an in-process measurement. Also, it is toilsome to prepare a plurality of surface profile measuring devices different in resolving power and measured range and to replace the surface profile measuring device in response to the object to be measured. Even if the measured object is the same, it will be substantially impossible to measure the same place. The economic loss is great.
Further, in such optical surface profile measuring devices using the critical angle method as is mentioned above, there has been a problem that, when the inclination of the measured surface is larger than a maximum of .+-.5.degree., the measurement will not be able to be made. Generally, in the case of using such surface profile measuring devices, there will be a requirement of obtaining a series of data while continuously measuring the surface of the object to be measured as described above. However, generally, on the surface of the object to be measured, such long periodic structures as waviness and chatter are present and the inclination of the surface to be measured can not be neglected in most cases. There has been a problem that, in such a case, due to the above mentioned problems, it will be difficult to continue the measurement. Further, there has been a problem that the set angle allowable range of the measured surface is so narrow that, in case such structure of the measured surface as is mentioned above is considered, in the case of beginning the measurement, a complicated setting operation will have to be made and such continuous measurement as in-process measurement will be made difficult. Even in the measurement of the profile of the inclined part of such a sample having step heights as an LSI pattern and in the inspection of the surface profile of such work inherently having an inclined structured on the surface as a superprecise bearing, high sensitivity measurement with no contact has come to be required. However, there has been also a problem that the conventional optical surface profile measuring device can not meet such multiple requirements.